Why does my engine hunt at idle?

Why does my engine hunt at idle?

I have a 1993 190E (2.3 4 cyl.) Regardless of outside conditions and how I’ve driven the car, when I stop, the idle fails to come down below 1800 to 2300 RPM and it hunts up and down between 1800 and 2300 RPM. f I take my foot off the brake, it accelerates and gets to whatever speed 1800 and 2300 RPM will allow. A few minutes later, everything goes back to normal RPM and all is well for a few days. Then, for no apparent reason, it does it again. M-B dealership has been unable to find the fault. I.we have tested all pneumatic hoses that have to do with idle, they replaced some temperature sensors and I’ve replaced the throttle body. I have a very good relationship with my dealership - have bought seven M-B cars there and I trust them. I wanted to buy another 190E from a private seller and that car drove normally except at the very end of the test drive. As I pulled into the seller’s driveway, that car did the same thing. The seller said that it had never done that before. I didn’t buy the car. But I’m wondering if anyone else has experienced this and has anyone diagnosed the problem successfully. Please don’t come back with lame “does the car have enough gas in the tank” types of suggestions. I am an aircraft mechanic and not a complete idiot around mechanical things. But this phenomenon baffles me as it does a dealership full of M-B mechanics. Thanks in advance for any help that you might be able to provide.

Asked by: Guest | Views: 231

Total answers/comments: 5

Comments display order:

bert [Entry]

"AndrommAT

Rep: 313

4

1"

bert [Entry]

"Have you checked the Idle Control Valve?

Also check vacuum hoses for leaks."

bert [Entry]

Check the wires from the air flow sensor…sometimes there is the isolation broken..

bert [Entry]

It is a standard electronic fuel injection running premium unleaded, as per the manual. Usually, when there’s a problem like this, it is an air leak around the “throttle body,” that is the plate in the intake path that is moved by the throttle i.e. accelerator pedal. This gives the air mass flow sensor the false impression that the throttle is open, that is, someone is pressing on the accelerator, which tells the ECU to increase fuel flow. Except in my case, all of the air paths have been checked and rechecked an hoses have been replaced. Also, since the ECU had been swapped with a known good one, that rules out the ECU sensing a “choke” condition i.e. low engine temperature and thereby increasing RPM. t also doesn’t answer the question of why does it do it intermittently? If there was an air leak, it would be there 24/7/365, not just once in a while.

bert [Entry]

"Hi there!

Did you check all the air hoses around the plenum? This sounds like an massive airleak of any sorts. What about the Rubber seals on the injectors? I would probably threw them all out and replace them. The old gasket on the Heater on the Plenum should also replaced. There are 2 gaskets on the head: the „normal“ paper gasket and the second metal heater gasket.

You should also think about looking for cracks in the rubber housing (underside) of that KE-Jet. There is also an Good Chance of an leak on the hoses around there.

good luck!"